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Geologic map of the Short Canyon quadrangle, Emery County, Utah 3
dicated the unit is the product of tidal flat deposition. Discussing
its deposition in the area of Capitol Reef National
Park (where outcrops are similar to those in Short Canyon
quadrangle), Smith and others (1963) judged that the Entrada
Sandstone was deposited in quiet water, and cited
Baker and others (1936) and Craig and others (1955),
who considered the deposition to have been largely subaqueous.
Curtis Formation
The Curtis Formation overlies the Entrada Sandstone in
the southeastern and eastern parts of the quadrangle. We
measured the Curtis in section 19, T. 22 S., R. 8 E., where
the formation is 214 feet (65 m) thick, and in sections 27
and 34, T. 21 S., R. 8 E., where the formation is 202 feet (62
m) thick. Craig and Dickey (1956) reported that the Curtis
is 250 feet (76 m) thick in the northern part of the San
Rafael Swell, but that it thins to the south and east. In the
Capitol Reef area to the south, the Curtis thins (Doelling
and Kuehne, 2007), and pinches out a few miles south of
the Wayne-Garfield county line.
In the Short Canyon quadrangle, the Curtis is mostly silty
sandstone with minor siltstone and mudstone (sandy).
At the base, it has a poorly sorted pebble conglomerate
as much as 0.5 foot (0.2 m) thick, with gray pebbles ranging
from coarse grit to 4 inches (10 cm) across, that are
contained in a matrix of fine- to coarse-grained sandstone.
The sandstone above is light gray, light green-gray, or light
tan-gray, moderately to poorly sorted, muddy, and mostly
calcareous. Most units are planar bedded; a few exhibit
faint cross-stratification. Siltstone, wherever present, is
medium gray in color. A few units, especially near the bottom
contain scattered pebbles.
The Curtis Formation consists of two informal units: a
lower mostly cliffy unit 95 to 105 feet (29–32 m) thick,
and an upper slope former about 85 to 100 feet (26–30
m) thick. The lower member (Jcl) forms a cliff with the
upper Entrada member (Jeu) and has a broad bench, partially
covered with mixed eolian and alluvial sand (Qea),
at its top (South Sand Bench). The lower 25 feet (8 m) of
the lower member is a green-gray calcareous siltstone and
sandy shale. It forms a slightly recessed vertical cliff and
locally a steep slope between the uppermost Entrada and
the cliffy sandstone. The cliffy part of the lower member
is mostly light-gray, fine- to medium-grained, calcareous
sandstone that is largely thin to medium bedded, but locally
thick to very thick bedded. The beds weather into nearly
structureless slabby planar blocks. The thicker beds locally
exhibit some eolian cross-bedding, knobby weathering,
and large tafoni ("stonepecker" holes).
The upper member (Jcu) exhibits a deeper green-gray,
light-brown, and lightpurplish
coloration. It displays a series of low slopes
capped by thin ledges. Although fine-grained sandstone
remains the dominant lithology, siltstone and mudstone
are more abundant than in the lower member. The slopes
are earthy or exhibit the thin-bedded to shaly nature of
the rock. The thin ledges weather into thin platy material,
pancake-shaped plates, or chips. The Curtis is one of the
easiest formations to identify in the quadrangle, forming
a light band between the brownish Entrada below and the
brownish Summerville Formation above.
The age of the Curtis Formation is early Late Jurassic. The
Curtis had been considered Callovian in age (see Hintze,
1988, p. 47), but recent palynological work by Wilcox and
Currie (2006) showed that it is early Oxfordian (earliest
Late Jurassic ~161 to 156 Ma). The Curtis Formation and
its correlatives were probably deposited in a single transgressive-regressive
sequence recording the final pulse of
the Jurassic interior seaway. The Curtis Formation correlates
with the Stump Sandstone of northern Utah. The Curtis
in the Short Canyon quadrangle is conformably overlain
by the Summerville Formation.
The Curtis sediments were deposited in a shallow-water
marine environment (Craig and Dickey, 1956), and some
of the sandstone is glauconitic. Glauconite is a cementing
mineral that forms only in marine environments where
sedimentation rates are relatively low. However, the moderate
to poor sorting in the lower part of the Curtis Formation
of the Short Canyon quadrangle might indicate some
wave action.
Summerville Formation
The Summerville Formation overlies the Curtis Formation
with a conformable, gradational contact in the Short Canyon
quadrangle. We measured it in section 19, T. 22 S., R.
8 E., northwest of the Canyon Pond where it is about 295
feet (90 m) thick. We also measured it north of Dry Wash
where it is 255 feet (80 m) thick. The Summerville Formation
was defined by Gilluly and Reeside (1928) at Summerville
Point in the northern San Rafael Swell where it
has a thickness of about 165 feet (50 m). In the San Rafael
Swell, the upper boundary with the Morrison Formation
is the J-5 unconformity, and the Summerville Formation
varies considerably in thickness due to gentle folding and
erosion prior to deposition of the Morrison. Although such
synclines and anticlines are not persistent throughout the
San Rafael Swell area, beveling of the formation beneath
the unconformity probably accounts for differences in
thickness of the Summerville. Trimble and Doelling (1978)
reported that the Summerville thickness in the San Rafael
River mining area, on the east side of the San Rafael Swell,
varies from 105 to 400 feet (32–122 m) because of smallamplitude
folds in the unit.
The Summerville Formation ordinarily forms a steep, con-